Miscellaneous fixes for building docs with Hadrian

Fix RankNTypes :ghc-flag: in users guide

This fixes Hadrian's build docs, the error being given was:

Error when running Shake build system:
  at want, called at src/Main.hs:89:30 in main:Main
* Depends on: docs
  at need, called at src/Rules/Documentation.hs:113:9 in main:Rules.Documentation
* Depends on: _build/docs/archives/users_guide.html.tar.xz
  at need, called at src/Rules/Documentation.hs:328:9 in main:Rules.Documentation
* Depends on: _build/docs/html/users_guide/index.html
* Raised the exception:
user error (Undefined reference targets found in Sphinx log.)

Another fix is included for the path to ghc-flags.txt, when running the compare-flags.py users guide script